Cancer includes a broad range of diseases, affecting approximately one in four individuals worldwide. The severity of the adverse impact of cancer cannot be understated, influencing medical policy and procedure as well as society generally. Because a hallmark of many types of cancer is rapid and unregulated proliferation of malignant cells, an overarching problem in improving approaches to cancer is the need for early detection and diagnosis. Numerous attempts have been made to develop accurate and reliable criteria for diagnosing the presence of a malignant condition. In particular, efforts have been directed to the use of serologically defined antigenic markers known as tumor associated antigens, which are either uniquely expressed by cancer cells or are present at markedly higher levels in subjects having a malignant condition.
However, due to the high heterogeneity of tumor associated antigen expression, for example the extreme diversity of carcinoma antigens, there is a need for additional tumor markers that are useful in cancer diagnosis. Many monoclonal antibodies reactive with carcinoma associated antigens are known (see, e.g., Papsidero, 1985 Semin. Surg. Oncol. 1:171, Allum et al., 1986 Surg. Ann. 18:41). These and other described monoclonal antibodies bind to a variety of different carcinoma associated antigens including glycoproteins, glycolipids and mucins (see, e.g., Fink et al., 1984 Prog. Clin. Pathol. 9:121; U.S. Pat. No. 4,737,579; U.S. Pat. No. 4,753,894; U.S. Pat. No. 4,579,827; U.S. Pat. No. 4,713,352). Many such monoclonal antibodies recognize tumor associated antigens that exhibit restricted expression on some but not other tumors originating in a given cell lineage or tissue type.
There are only relatively few examples of tumor associated antigens that appear to be useful for identifying a particular type of malignancy. Monoclonal antibody B72.3, for example, specifically binds to a high molecular mass (>106 Da) tumor-associated mucin antigen that is selectively expressed on a number of different carcinomas, including most if not all ovarian carcinomas and an overwhelming majority of non-small cell lung carcinomas, colon carcinomas and breast carcinomas (see, e.g., Johnston, 1987 Acta Cytol. 1:537; U.S. Pat. No. 4,612,282). Nevertheless, detection of cell-associated tumor markers such as the mucin antigen recognized by B72.3 following surgical resection of a tumor may be of limited usefulness for diagnostic screening, in which early detection of a malignant condition prior to accumulation of substantial tumor mass is preferred.
An alternative to the diagnosis of a particular type of cancer by screening surgically resected specimens for tumor associated antigens, where invasive surgery is usually indicated only after detection of an accumulated tumor mass, would be to provide compositions and methods for detecting such antigens in samples obtained from subjects by non-invasive or minimally invasive procedures. In ovarian and other carcinomas, for example, there are currently a number of soluble tumor associated antigens that are detectable in samples of readily obtained biological fluids such as serum or mucosal secretions. One such marker is CA125, a carcinoma associated antigen that is also shed into the bloodstream, where it is detectable in serum (e.g., Bast et al., 1983 N. Eng. J. Med. 309:883; Lloyd et al., 1997 Int. J. Canc. 71:842). CA125 levels in serum and other biological fluids have been measured along with levels of other markers, for example, carcinoembryonic antigen (CEA), squamous cell carcinoma antigen (SCC), tissue polypeptide specific antigen (TPS), sialyl TN mucin (STN) and placental alkaline phosphatase (PLAP), in efforts to provide diagnostic and/or prognostic profiles of ovarian and other carcinomas (e.g., Sarandakou et al., 1997 Acta Oncol. 36:755; Sarandakou et al., 1998 Eur. J. Gynaecol. Oncol. 19:73; Meier et al., 1997 Anticanc. Res. 17(4B):2945; Kudoh et al., 1999 Gynecol. Obstet. Invest. 47:52; Ind et al., 1997 Br. J. Obstet. Gynaecol. 104:1024; Bell et al. 1998 Br. J. Obstet. Gynaecol. 105:1136; Cioffi et al., 1997 Tumori 83:594; Meier et al. 1997 Anticanc. Res. 17(4B):2949; Meier et al., 1997 Anticanc. Res. 17(4B):3019).
Elevated levels of serum CA125 alone or in combination with other known indicators, however, do not provide a definitive diagnosis of malignancy, or of a particular malignancy such as ovarian carcinoma. For example, elevated CA125, CEA and SCC in vaginal fluid and serum correlate most strongly with inflammation in benign gynecological diseases, relative to cervical cancer and genital tract cancers (e.g., Moore et al., 1998 Infect. Dis. Obstet. Gynecol. 6:182; Sarandakou et al., 1997 Acta Oncol. 36:755). As another example, elevated serum CA125 may also accompany neuroblastoma (e.g., Hirokawa et al., 1998 Surg. Today 28:349), while elevated CEA and SCC, among others, may accompany colorectal cancer (Gebauer et al., 1997 Anticanc. Res. 17(4B):2939). Thus the compelling need for additional markers to be used, including markers useful in multi-factor diagnostic screening, is apparent. (See, e.g., Sarandakou et al., 1998; Kudoh et al., 1999; Ind et al., 1997.)
The differentiation antigen mesothelin is expressed on the surfaces of normal mesothelial cells and also on certain cancer cells, including epithelial ovarian tumors and mesotheliomas. Also known as CAK1, mesothelin is identified by its reactivity with the monoclonal antibody K-1 (MAb K-1), which was generated following immunization with the OVCAR-3 ovarian carcinoma cell line (Chang et al., 1992 Canc. Res. 52:181; Chang et al., 1992 Int. J. Canc. 50:373; Chang et al., 1992 Int. J. Canc. 51:548; Chang et al., 1996 Proc. Nat. Acad. Sci. USA 93:136; Chowdhury et al., 1998 Proc. Nat. Acad. Sci. USA 95:669). Mesothelin is synthesized as an approximately 70 kDa glycoprotein precursor having a C-terminal glycosylphosphatidylinositol (GPI) linkage site for cell membrane attachment. This precursor is processed by, inter alia, proteolytic cleavage into at least two components: (i) a shed N-terminal ˜31 kDa polypeptide (Chowdhury et al., 1998 Proc. Nat. Acad. Sci. USA 95:669) having extraordinarily high homology to a soluble 31 kDa polypeptide known as megakaryocyte potentiating factor (MPF) that is similarly derived by proteolysis of an approximately 70 kDa GPI-linked glycoprotein precursor belonging to the mesothelin polypeptide family (Yamaguchi et al., 1994 J. Biol. Chem. 269:805; Kojima et al., 1995 J. Biol. Chem. 270:21984; and (ii) a mature 40 kDa GPI-linked, cell surface-bound C-terminal mesothelin glycosylated polypeptide, which bears the K-1 (MAb K-1) recognition epitope (Chang et al., 1996). As defined by reactivity with MAb K-1, mesothelin is present on a majority of squamous cell carcinomas including epithelial ovarian, cervical and esophageal tumors, and on mesotheliomas (Chang et al., 1992 Canc. Res. 52:181; Chang et al., 1992 Int. J. Canc. 50:373; Chang et al., 1992 Int. J. Canc. 51:548; Chang et al., 1996 Proc. Nat. Acad. Sci. USA 93:136; Chowdhury et al., 1998 Proc. Nat. Acad. Sci. USA 95:669). Using MAb K-1, mesothelin is detectable only as a cell-associated tumor marker and has not been found in serum from ovarian cancer patients, or in medium conditioned by OVCAR-3 cells (Chang et al., 1992 Int. J. Cancer 50:373). Thus mesothelin, despite an expression pattern that correlates with specific malignant conditions, does not appear to offer a useful marker for early diagnostic screening, because only cell-associated and not soluble forms of mesothelin may be detectable by known methods.
The compositions and methods of the present invention overcome these limitations of the prior art by providing a method of screening for the presence of a malignant condition using antibodies specific for mesothelin/MPF and/or mesothelin/MPF-related antigens to detect polypeptides that naturally occur in soluble form, and offer other related advantages.